Composition based on zirconium oxide and oxides of cerium, lanthanum and of another rare earth, a method for preparing same and use thereof as catalyst

ABSTRACT

The invention concerns a composition based on zirconium and cerium oxides in an atomic ratio Zr/Ce&gt;1, and further comprising lanthanum oxide or an oxide of a rare earth other than cerium and lanthanum. The invention is characterized in that after calcination for 6 hours at 1150 .C it has a specific surface area of not less than 10 m; /g. The composition is obtained by forming a mixture containing a sol of a zirconium compound and cerium, lanthanum, said rare earth compounds, contacting said mixture with a basic compound solution, while heating and calcining the resulting precipitate. The composition can be used as catalyst.

The present invention relates to a composition based on zirconium oxideand oxides of cerium, lanthanum and another rare earth, its method ofpreparation and its use as a catalyst.

So-called multifunctional catalysts are used at present for thetreatment of the exhaust gases from internal combustion engines(automotive post-combustion catalysis). “Multifunctional” meanscatalysts that are able to effect not only the oxidation in particularof the carbon monoxide and hydrocarbons present in the exhaust gases butalso the reduction in particular of the nitrogen oxides that are alsopresent in these gases (“three-way” catalysts). Zirconium oxide andcerium oxide are now seen as two particularly important and advantageousconstituents for catalysts of this type. To be effective, thesecatalysts must have a large specific surface even at elevatedtemperature.

There is a need for catalysts that can be used at higher and highertemperatures and, hence, with great stability of their specific surface.

The object of the invention is therefore the development of a catalyticcomposition that can meet this need.

For this purpose, the composition of the invention is based on zirconiumoxide and cerium oxide in an atomic ratio Zr/Ce>1, in addition itcomprises lanthanum oxide and an oxide of a rare earth other than ceriumand lanthanum, and it is characterized in that after calcination for 6hours at 1150° C. it has a specific surface of at least 10 m²/g.

The invention also relates to a method of preparation of theaforementioned composition and this method is characterized in that itcomprises the following stages:

-   a mixture is prepared comprising compounds of cerium, of lanthanum    and of the aforementioned rare earth and a sol of a zirconium    compound;-   said mixture is brought into contact with a solution of a basic    compound whereby a precipitate is obtained;-   said precipitate is heated in an aqueous medium;-   the precipitate thus obtained is calcined.

As mentioned previously, the composition of the invention hasparticularly high values of specific surface at a temperature of 1150°C.

Other characteristics, details and advantages of the invention willbecome even clearer on reading the description given below, as well as aconcrete but nonlimiting example that is intended to illustrate it.

Hereinafter, specific surface means the BET specific surface determinedby nitrogen adsorption according to standard ASTM D 3663-78 establishedon the basis of the Brunauer-Emmett-Teller method described in TheJournal of the American Chemical Society, 60, 309 (1938).

Moreover, the calcinations following which the area values are given arecalcinations in air.

“Rare earth” means the elements of the group comprising yttrium and theelements of the Periodic Table with atomic number between 57 and 71inclusive.

The compositions of the invention are based on zirconium oxide and inaddition they contain oxides of three other elements. These elements arecerium, lanthanum and a third rare earth that is different from ceriumand lanthanum. Quite particularly, this third rare earth can beneodymium.

The compositions of the invention are further characterized by theirspecific surface after high-temperature calcination. Thus, aftercalcination for 6 hours at 1150° C., said specific surface can be atleast 10 m²/g, more particularly at least 15 m²/g.

Said surface still has significant values after calcination for 6 hoursat 1200° C., namely at least 3 m²/g.

Depending on the embodiments, the compositions of the invention can alsoexhibit large surface areas at 900° C. after 6 hours of calcination, forexample at least 50 m²/g, more particularly at least 70 m²/g and evenmore particularly at least 75 m²/g. At 1000° C., after 6 hours ofcalcination, said area can be at least 40 m²/g, more particularly atleast 55 m²/g.

Furthermore, after calcination for 6 hours at 1100° C., the compositionsof the invention can possibly, depending on the embodiments, have aspecific surface of at least 20 m²/g.

According to a particular embodiment, the compositions of the inventionmay be in the form of a pure solid solution of the oxides of cerium, oflanthanum and of the other rare earth in the zirconium oxide. This meansthat the cerium, the lanthanum and the other rare earth are entirelypresent in solid solution in the zirconium. The X-ray diffractionspectra of these compositions reveal in particular, within saidcompositions, the presence of a unique, clearly identifiable phasecorresponding to that of a zirconium oxide crystallized in the cubic orquadratic system, thus reflecting the incorporation of the cerium,lanthanum and other rare earth in the crystal lattice of the zirconiumoxide, and therefore the production of a true solid solution.

In this embodiment the solid solution phase is stable. By this we meanthat following the calcinations at 900° C. but also at 1000° C. for saidtimes the compositions of the invention are still in the form of thisunique phase. Moreover, and according to a more particular embodiment,they can still have this structure of pure solid solution even aftercalcination for 6 hours at 1100° C. In other words, no demixing isobserved in the temperature range from 900° C. to 1100° C.

The contents of the various elements in the compositions can vary. Saidcontents are expressed hereinafter as weight of oxide (ZrO₂, CeO₂,TR₂O₃, where TR denotes lanthanum and the other rare earth). Generally,the zirconium content is at least 50%, more particularly at least 60%and even more particularly at least 70%. The cerium content is generallyless than 50%, more particularly 40% at most, and even more particularly25% at most. The lanthanum content is usually 5% at most and moreparticularly it can be between 1% and 3%. Finally, the content of therare earth can be 15% at most and more particularly it can be between 3%and 10%. In the case of compositions in the form of solid solutions, theupper limits of the contents of lanthanum and third rare earth are infact only imposed for just the limit of solubility of these species inzirconium oxide.

Another characteristic of the compositions of the invention is that theyare sulfur-free. By this we mean that the sulfur content is below 200ppm, and preferably below 100 ppm. Said content is expressed as weightof sulfate (SO₄) relative to the whole composition.

The method of preparation of the compositions of the invention will nowbe described.

The first stage of this method comprises preparing a mixture comprisingcompounds of cerium, of lanthanum and of the third rare earth on the onehand and a sol of a zirconium compound on the other hand.

This mixture is usually prepared in an aqueous medium.

“Sol” means any system consisting of fine solid particles of colloidaldimensions, i.e. of dimensions between about 1 nm and about 500 nm,based on a zirconium compound, said compound generally being an oxideand/or hydroxide of zirconium, suspended in an aqueous liquid phase,said particles being able in addition, optionally, to contain residualamounts of bound or adsorbed ions, such as for example nitrates,acetates, chlorides or ammonium ions. It should be noted that, in saidsol, the zirconium can be either completely in the form of colloids orin the form of ions and of colloids simultaneously.

The starting sol can be obtained notably by heat treatment or hothydrolysis of a solution of zirconium oxychloride (ZrOCl₂). Thistreatment is generally carried out at a temperature of at least 80° C.and which can be between about 100° C. and 300° C., and preferablybetween 120° C. and 200° C., the concentration of the solution ofzirconium oxychloride preferably being between 0.1 and 3 mol/l, moreparticularly between 0.5 and 2 mol/l expressed as ZrO₂.

The zirconium sol can also be obtained by the action of nitric acid on ahydroxide or carbonate of zirconium. To obtain a sol in the sense givenabove, this treatment with nitric acid must be carried out in specificconditions. Thus, the molar ratio NO₃ ⁻/Zr must be between about 1.7 andabout 2.3 in the case of a hydroxide and about 1.7 and about 2 in thecase of a carbonate. Above the maximum values of this ratio there is arisk of not obtaining colloids. Below the minimum value of this ratio,there is a risk that the properties of stability of surface area of thecompositions will not be obtained.

Notably it is possible to use zirconium sols having an average size ofcolloids between 5 nm and 500 nm, and advantageously between 10 and 200nm (The size or average hydrodynamic diameter is as determined byquasi-elastic diffusion of light according to the method described byMichael L. McConnell in the journal Analytical Chemistry 53, No. 8, 1007A, 1981).

As compounds of cerium, of lanthanum and of rare earth that can be usedin the method of the invention, we may mention for example the salts ofinorganic or organic acids, notably of the sulfate, nitrate, chloride oracetate type. More particularly it is possible to use salts of cerium IVsuch as ceric nitrate or ammonium ceric nitrate. It should be noted thatthe nitrates are in general particularly suitable.

The amounts of zirconium, cerium, lanthanum and rare earth in themixture must correspond to the stoichiometric proportions required forobtaining the desired final composition.

The second stage of the method comprises bringing the mixture obtainedin the first stage into contact with a solution of a basic compound.

As basic compound we may mention products of the hydroxide or carbonatetype. We may mention the hydroxides of alkali metals or of alkalineearths. It is also possible to use secondary, tertiary or quaternaryamines. However, the amines and ammonia may be preferred since theylessen the risks of pollution by alkaline or alkaline-earth cations.Urea may also be mentioned.

The manner in which the mixture is brought into contact with thesolution, i.e. their order of introduction, is not critical. However,this bringing into contact can be effected by introducing the mixtureinto the solution of the basic compound. This variant is preferable forobtaining the compositions in the form of solid solutions.

Finally it should be noted that, when the starting mixture contains acerium compound in which the cerium is in the form of Ce III, it ispreferable to use an oxidizing agent, for example hydrogen peroxide,during the procedure. This oxidizing agent can be used by being added tothe reaction medium during this second stage.

The bringing into contact or reaction between the mixture and thesolution, notably addition of the mixture to the solution of the basiccompound, can be effected in one go, gradually or continuously, and itis preferably carried out with stirring. It is preferably carried out atroom temperature. Finally, the reaction is carried out in conditionssuch that the pH of the medium formed is at least 7, and moreparticularly at least 9.

The next stage of the method is the stage of heating the precipitate inan aqueous medium.

This heating can be carried out directly on the reaction medium obtainedafter reaction with the basic compound or on a suspension obtained afterseparation of the precipitate from the reaction medium, washing ifnecessary and putting the precipitate back into water. The temperatureto which the medium is heated is at least 40° C., more particularly atleast 60° C. and even more particularly at least 100° C. The medium isthus maintained at a constant temperature for a time that is usually atleast 30 minutes and more particularly at least 1 hour. Heating can becarried out at atmospheric pressure or optionally at a higher pressure.

The medium submitted to heating is preferably at basic pH.

It is possible to carry out several heating operations. Thus, theprecipitate obtained after the stage of heating and optionally ofwashing can be resuspended in water then another heating of the mediumthus obtained can be carried out. This other heating is carried out inthe same conditions as those described for the first.

In a last stage of the method according to the invention, theprecipitate recovered, optionally after washing and/or drying, can thenbe calcined. This calcination makes it possible to develop thecrystallinity of the product formed, and it can also be adjusted and/orselected as a function of the intended temperature of subsequent use ofthe composition according to the invention, taking into account that thespecific surface of the product decreases as the calcination temperatureemployed is raised. Said calcination is generally effected in air, butcalcination carried out for example in inert gas or in a controlledatmosphere (oxidizing or reducing) is not of course excluded.

In practice, generally the calcination temperature is limited to a rangeof values between 300 and 1000° C.

The compositions of the invention as described above or as obtained inthe method investigated previously are in the form of powders but theycan optionally be formed into granules, spheres, cylinders or honeycombsof various sizes.

The invention also relates to catalytic systems comprising thecompositions of the invention. For said systems, these compositions canthus be applied to any support usually used in the area of catalysis,i.e. notably thermally inert supports. Said support can be selected fromalumina, titanium dioxide, cerium oxide, zirconium oxide, silica,spinels, zeolites, silicates, crystalline silicon-aluminum phosphates,crystalline aluminum phosphates.

The compositions can also be used in catalytic systems comprising a washcoat with catalytic properties and based on these compositions, on asubstrate of the monolithic metallic type or of ceramic, for example.The wash coat can itself also include a support of the type of thosementioned above. This wash coat is obtained by mixing the compositionwith the support so as to form a suspension which can then be depositedon the substrate.

These catalytic systems and more particularly the compositions of theinvention can find numerous applications. They are thus particularlysuitable for, and can therefore be used in, the catalysis of variousreactions such as, for example, dehydration, hydrosulfurization,hydrodenitrification, desulfurization, hydrodesulfurization.,dehydrohalogenation, reforming, steam reforming, cracking,hydrocracking, hydrogenation, dehydrogenation, isomerization,disproportionation, oxychlorination, dehydrocyclization of hydrocarbonsor other organic compounds, reactions of oxidation and/or reduction, theClaus reaction, treatment of the exhaust gases of internal combustionengines, demetalization, methanation, shift conversion.

In the case of these uses in catalysis, the compositions of theinvention are used in combination with precious metals. The nature ofthese metals and the techniques of incorporating the latter in thesecompositions are familiar to a person skilled in the art. For example,the metals can be platinum, rhodium, palladium or iridium, and they cannotably be incorporated in the compositions by impregnation.

Among the uses mentioned above, treatment of the exhaust gases ofinternal combustion engines (automotive post-combustion catalysis)represents a particularly interesting application. Accordingly, theinvention also relates to a method of treatment of the exhaust gases ofinternal combustion engines that is characterized in that a catalyticsystem as described above or a composition according to the inventionand as described previously is used as the catalyst.

An example will now be given.

EXAMPLE

This example relates to the synthesis of an oxide with the compositionZrO₂/CeO₂/La₂O₃/Nd₂O₃ with respective proportions by weight of73.5/20/2.5/4.

Starting Materials Used:

The concentrations C are expressed as oxide. Solution of Ce(NO₃)₃ C =29.2% d = 1.718 g/cm³ Solution of La(NO₃)₃ C = 29.1% d = 1.775 g/cm³Solution of Nd(NO₃)₃ C = 26.7% d = 1.682 g/cm³ NH₄OH at 20% (Prolabo)ZrOCl₂ C = 24.6% as ZrO₂

Preparation of the Zirconium Sol A solution of zirconium oxychlorideC=24.6% as ZrO₂ is prepared first. The solution is then treated in anautoclave at 160° C. for 8 hours with stirring (80 rev/min). Thesuspension thus obtained is centrifuged at 3500 rev/min then peptized(the content of zirconium oxide is 38%).

Preparation of the Oxide ZrO₂/CeO₂/La₂O₃/Nd₂O₃

The zirconium sol thus synthesized is dispersed in 350 ml of water andthen all of the solutions of cerium, lanthanum and neodymium nitrate areadded to it with stirring for 10 minutes.

In parallel, 500 ml of ammonia solution is placed in a 1-liter reactor.The previously prepared suspension is added to this solution at a flowrate of 10 ml/min. At the end of addition the pH is 10.5. Theprecipitate is centrifuged (v=4500 rev/min) then resuspended in 760 mlof ammonia water at pH=10.5 and centrifuged again. The operation isrepeated three times. The cake thus obtained is resuspended in 760 ml ofammonia water and is stirred (v=300 rev/min) for 2 hours at 150° C.After cooling, the suspension is centrifuged then washed in the presenceof ammonia water (pH=10.5) in the conditions described previously. Thisoperation is repeated three times then the cake obtained is resuspendedat a concentration of 100 g/l of oxide and atomized with the Buchi®. Theair inlet and outlet temperatures of the Buchi® are equal to 250° C. and110° C. respectively.

The dried solid is then calcined in a muffle furnace for 4 h at 900° C.(rate of temperature rise 1° C./min).

Specific Surfaces

The surface areas of the product obtained after calcination at differenttemperatures are shown below.

4 h 900° C.: 77 m²/g; 10 h 1000° C.: 55 m²/g; 10 h 1100  C: 23 m²/g; 10h 1150° C.: 16 m²/g; 10 h 1200  C.: 3.5 m²/g.

The X-ray diffraction pattern of the composition after calcination at900° C. and 1000° C. shows that it is in the form of a solid solutioncorresponding to a tetragonal zirconium oxide.

1-16. (canceled)
 17. A composition based on zirconium oxide comprisingcerium oxide in an atomic ratio Zr/Ce>1, and in addition comprisinglanthanum oxide and an oxide of a rare earth other than cerium andlanthanum, wherein after calcination for 6 hours at 1150° C. it has aspecific surface of at least 10 m²/g.
 18. The composition as claimed inclaim 17, wherein after calcination for 6 hours at 1150° C. it has aspecific surface of at least 15 m²/g.
 19. The composition as claimed inclaim 17, wherein after calcination for 6 hours at 1200° C. it has aspecific surface of at least 3 m²/g.
 20. The composition as claimed inclaim 17, wherein after calcination for 6 hours at 900° C. it has aspecific surface of at least 50 m²/g.
 21. The composition as claimed inclaim 17, wherein after calcination for 6 hours at 1000° C. it has aspecific surface of at least 40 m²/g.
 22. The composition as claimed inclaim 17, wherein the rare earth is neodymium.
 23. The composition asclaimed in claim 17, wherein the contents by weight of oxides are atleast 50% for zirconium, less than 50% for the oxide of cerium, 5% atmost for lanthanum and 15% at most for the rare earth.
 24. Thecomposition as claimed in claim 17, being sulfur-free.
 25. A method ofpreparation of a composition as claimed in claim 17, comprising thesteps of: a) preparing a mixture comprising compounds of cerium, oflanthanum and of the aforementioned rare earth and a sol of a zirconiumcompound; b) adding to the mixture of step a) a solution of a basiccompound whereby a precipitate is obtained; c) heating said precipitatein an aqueous medium; and d) calcining the precipitate thus obtained instep c).
 26. The method as claimed in claim 25, wherein the sol of azirconium compound of step a) is obtained by heat treatment of anaqueous solution of a zirconium oxychloride.
 27. The method as claimedin claim 25, wherein the sol of a zirconium compound of step a) isobtained by the action of nitric acid on a hydroxide or carbonate ofzirconium in a molar ratio NO₃ ⁻/Zr between 1.7 and 2.3 in the case of ahydroxide and 1.7 and 2 in the case of a carbonate.
 28. The method asclaimed in claim 25, wherein in step c) the precipitate is heated at atemperature of at least 100° C.
 29. The method as claimed in claim 25,wherein in step c) the heating of the precipitate is carried out atbasic pH.
 30. A catalytic system, comprising a composition as defined inclaim
 17. 32. A method of treatment of the exhaust gases of internalcombustion engines, comprising the step of treating said gases with acatalytic system as claimed in claim 30 or a composition as claimed inclaim 17.